marker-assisted selection in wheat - ictsd

104Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishtolerance to inbreeding depression must beexerted. However, such selection is biasedby the differences in homozygosity levelsof segregating partially inbred genotypes.This highlights the need for a methodto measure the level of heterozygosityin these partially inbred individuals andto use this in a co-variance correction inthe selection of phenotypically vigorousgenotypes. Molecular markers can be usedto estimate the level of homozygosity ofa given plant, enabling selection of plantswith true tolerance to inbreeding.Molecular markers can identify regionsin the genome that are particularly relatedto the expression of heterosis and for measuringgenetic distances among inbred linesto direct crosses with higher probabilities ofhigh heterosis. Co-dominant SSR markerson a genome-wide basis are suitable forthis purpose. The effect of self-pollinationon vigour and heterozygosity was analysedin nine S 1 families, heterozygosity beingestimated in the S 1 families by 100 mappedSSR markers that cover over 80 percent ofthe cassava genome and plant vigour by dryroot yield and plant biomass. Results willassist in selecting the best performing andleast heterozygous plants during inbreedingby identifying superior partially inbredparental lines. Molecular markers couldalso be used to delineate heterotic groups incassava. Genetic resources of cassava havebeen characterized at the regional (Fregeneet al., 2003) and global (Hurtado et al.,2005) levels. Highly differentiated groupsof accessions were observed particularlyamong groups of materials from Guatemalaand Africa and they may represent heteroticpools. These groupings are being testedbased on molecular markers by geneticcrossing between and within the groupsas a first step to define heterotic patternsfor a more systematic improvement ofcombining ability via recurrent reciprocalselection.Other potential MAS targetsSeveral other traits for which MAS can beapplied to increase efficiency of breedinginclude:Beta-caroteneCIAT and a number of partners are involvedin a project to produce cassava varieties withhigher levels of β-carotene in yellow roots.This is one way of combating the deficiencyof this key micronutrient in areas wherecassava is a major staple. The experimentalapproach to increasing cassava β-carotenecontent includes conventional breeding andgenetic transformation. The discovery of awide segregation pattern of root colour intwo S 1 families from the Colombian landraceMCOL 72 (cross code AM 273) and MTAI8 (AM 320) was the basis for moleculargenetic analysis of β-carotene content incassava. Three markers, SSRY251, NS980and SSRY330, were found to be associatedwith β-carotene content. These are in thesame region of the genome and togetherexplain >80 percent of phenotypic variationfor β-carotene content in the populationused for this study. The homozygous stateof certain alleles of these markers translatesinto higher β-carotene content, suggestingthat breeding for this trait can benefit frommolecular markers to assist in combiningfavourable alleles in breeding populations.The work is continuing with the searchfor additional favourable alleles in yellowrootedgermplasm to give the best possiblephenotypic expression of the trait.Cyanogenic potentialA collaborative project between the SwedishUniversity of Agricultural Sciences (SLU),Uppsala, the Medical Biotechnology